With advancements in technology, the transmission of voice and data at faster rates and in larger volumes is always in demand. One solution to fulfilling these demands is digital subscriber line (DSL) technology. DSL technology has been introduced into the field of broadband networking, among other reasons, to overcome issues faced by traditional voice band technology. Such issues include, but are not limited to, bandwidth limitations. Multiple DSL technologies exist including, but not limited to, rate adaptive DSL (RADSL), symmetric DSL (SDSL), multi-rate SDSL (M/SDSL), high bit-rate DSL (HDSL), very high bit-rate DSL (VDSL), and asymmetric DSL (ADSL).
ADSL technology utilizes the infrastructure already in place in a public switched telephone network (PSTN), including copper loops, constructed of copper wires, between a customer premise and a central office. Advantageously, ADSL technology does not require replacement of network equipment such as routers, switches, firewalls and Web servers, which are commonly used in today's paradigm for broadband access.
The American national standards institute (ANSI) standardizing body selected discrete multi-tone (DMT) as the modulation scheme for ADSL. DMT is a special implementation of multi-carrier modulation that is based on the discrete Fourier transform, which can conveniently be implemented in a fully digital manner.
Impulse noise is a factor that is considered in the fabrication of DSL systems having DMT transceivers. Specifically, impulse noise, or a burst, is an unwanted disturbance of a relatively short duration that typically results from energy that is coupled from an electrical transient located near DMT transceivers.
To address and alleviate errors caused by impulse noise, and other factors that negatively effect data transmission in a DSL system, typical error correction techniques utilize a redundancy approach in which bits of data are repeated a number of times before transmission. Unfortunately, most redundancy techniques defeat the purpose of high speed and high density data processing and transmission since excessive amounts of data bits are added to a data bit stream to provide for error correction.
Reed Solomon (RS) coding and interleaving are important techniques used in a DMT transceiver that provide error correction with the addition of a minimal number of data bits. RS coding is an error correction code that is widely used due to its relatively large error correction capability when weighed against the minimal added overhead imposed upon a data transmission system. RS codes are an example of a block coding technique where the data bit stream to be transmitted is broken up into RS blocks and redundant data bits are then added to each block. The size of these blocks and the amount of check data bits added to each block is either specified for a particular application or can be user-defined for a closed system. Within each block, data is further divided into a number of symbols that are generally from six to ten bits in size.
An interleaver spreads the error caused by impulse like noise over the number of RS blocks such that each of the frames contains only a small number of error bits. Therefore, the number of DMT symbols in each RS frame is increased. The small number of error bits added can then be corrected by a RS decoder.
Unfortunately, while the longer the interleaving depth (D) is, the better the protection is provided against impulse-like noise, longer interleaving depth also provides longer delay in data transmission. In addition, particular for DMT transceivers, the number of DMT symbols contained in each RS frame is important because the number of DMT symbols per frame also affects delay and payload rate. Therefore, it is necessary to perform a balancing test to allow for interleaving and RS coding, while not excessively delaying data transmission.